This invention relates to a switching type PC-PC converter, and more particularly to means for increasing conversion efficiency at light load.
In a switching type PC-PC converter, a primary DC power supply is switched by a switching element for generating an alternating current which flows in a primary winding of a transformer. An alternating current voltage induced in a secondary winding of the transformer is rectified to produce a secondary DC voltage which has a voltage value different from that of the primary DC power supply.
Usually, load current of such PC-PC converter varies in a wide range, and the apparatus is often used at a light load for a long time. Therefore, high conversion efficiency at light load is desired for such a DC-DC converter.
There have been several proposals of an apparatus for increasing light load efficiency of such PC-PC converters. For example, in a Japanese Patent Application No. 309585/'89 (laid open as No. 173352/'91, called Ref. 1 in this specification), the switching of the primary DC power supply is operated intermittently during light load period. During a quiescent phase of the switching, DC current is supplied to a load by a capacitor connected in parallel to the load. Therefore, the capacitor must have a fairly large capacity and this makes an obstacle against miniaturization of the apparatus. And in Ref. 1, light load is detected by a rise of an output voltage. This is not to be recommended when a good voltage regulation is required for the apparatus. Moreover, when the load current is abruptly increased in the apparatus of Ref. 1, the output voltage will suddenly drop due to a sudden discharge of the capacitor and will require a fairly long time to recharge the capacitor even when the switching circuit is instantly changed to a full time operation from the intermittent operation.
In another Japanese Patent Application No. 73555/'89 (laid open as No. 254972/'90, called Ref. 2 in this specification), a snubber capacitor is decreased in capacity during a light load period. In an ordinary type DC-DC converter., a capacitor called a snubber capacitor is connected in parallel to the switching element for protecting the switching element from a surge voltage generated by a primary current in the primary winding of the transformer and the leakage inductance of the winding. When the load current is decreased in a light load period, the primary current is also decreased, and a smaller capacity of the snubber capacitor will be sufficient for protecting the switching element. A voltage on the snubber capacitor charged during an off-phase of the switching, is discharged during an on-phase of the switching, making a power loss independent of the load current. This power loss is decreased when the capacity of the snubber capacitor is decreased. In Ref. 2, the light load is detected by a decrease of surge peak voltage across the snubber capacitor, and this decrease of the peak voltage automatically disconnects a part of the snubber capacitor.
In an apparatus of Ref. 2, only a part of the power loss independent of the load current is decreased, and even when all the snubber capacitor is removed, there remains a stray capacity in parallel to the switching element. Charging and discharging of this stray capacity makes a substantial loss in a light load period.